It has been known for a long time to record and read data on a magnetic tape by means of an apparatus having a rotating head.
It is known that information recorded with the aid of apparatus of this kind is carried on the tape along linear segments or on tracks, of which some are longitudinal (reserved for example for recording audible signals and/or synchronization marks) and others are oblique relative to the longitudinal axis of the of the tape and receive the data or information recorded for the purpose of subsequent processing.
Likewise in known manner, the use of multitrack recorders enables the throughput of information recorded or read on the tape to be increased in proportion to the number of tracks.
Nevertheless, there are limitations to the information throughput which can be achieved, in particular because of the difficulties of the processing in real time of the information read back on the tape.
In certain applications, for example, digital information representing physical paramaters and coming from a plurality of sensors is recorded in real time at a high rate. This information is intended to be read back subsequently and processed in deferred time by a computer.
If the information read rate cannot be modified relative to the information record rate, the latter cannot then exceed the maximum value of the rate acceptable to the computer effecting the processing of the information read back from the tape, which is bothersome when the number of data to be recorded in real time per unit of time is high.
In other cases it may be that the computer has to work under conditions in which its minimum acceptable rate is higher than the information record rate.
It is then desirable to be able to vary, in the highest possible proportions, the ratio of information read and record rates.
Two parameters must be maintained between recording and reading, namely on the one hand the trajectory of the head relative to the tape, so as to read back the information at the exact point where it was recorded, and on the other hand the relative speed of the head and the tape, which cannot be modified by more than a few hundredths in relative value without leading to distorsions of the signal.
However, since the speed of the heads is much (higher than the speed of the tape, the speed of the latter can be modified in order to obtain a variation of the information rate, without entailing an unacceptable variation of the relative speed of the head in relation to the tape.
Means are already known for making it possible with the aid of a rotary head to read back information on a magnetic tape whose speed may assume two different values. In particular, means of this kind are provided in video recorders which permit picture freeze and in which the heads can scan either the successive tracks of the moving magnetic tape or a single track of the motionless tape.
In order to compensate for the variation of inclination of the trajectory of the heads in relation to the tape, which occurs when the tape speed passes from its nominal value to zero value or vice versa, in these known devices the trajectory of the heads is corrected in such a manner that the heads continue to follow the track or tracks corresponding to the information recorded.
Nevertheless, in these known devices the correction is of low amplitude and, as is shown for example in U.S. Pat. No. 4 410 918, can be effected with the aid of a piezoelectric bimetallic strip which at all times controls the position of each head.
However, a solution of this kind entails the use of sliding contacts whose operation is delicate and whose reliability is only moderate, and the use of which becomes expensive and complicated when there are a large number of heads. Moreover, this solution can be applied easily only when the correction of trajectory is of low amplitude.